The new method for examining nuclei in 3D allows one to hybridize to whole tissue, remove the unhybridized labeled probe and then process the tissue for sectioning. We cut 1 micron sections with a glass knife, getting 10 to 15 slices through nuclei. Autoradiographs were prepared and after 2 to 3 weeks of exposure (3H-1.705 DNA as the probe) the images were developed. Photography of the 10 to 15 nuclear sections allows one to reconstruct an accurate 3-D composite of various nuclear types in Drosophila Malpighian tubes and salivary gland nuclei. We are now ready to do many other probes, one at a time, two at a time and even three if the genes are separated enough. The histone genes in man are located on chromosome 7 at 7q2 in the middle of the long arm, determine in a man-mouse hybrid cell line. The probe was 3H-cRNA made from all five of the histone genes of the sea urchin. Several control chromosome confirmed chromosome 7 as the histone gene site. The histone genes in maize are probably on the short arm of chromosome 8. These are being confirmed cytogenetically using translocations this summer. Again in Zea mays we mapped the 5S rRNA genes at or near .82 on 2L using translocation heterozygotes. In our studies with human chromosomes the estimate of the number of 5S rRNA genes on chromosome one at 1q42-43 is 73 per haploid genome, considerable below estimates made nearly 10 years ago. The number of human 28S rRNA gene is 37 per haploid genome. Since chromosomes 21 and 22 usually have twice as many rDNA copies as 13, 14, and 15, chromosomes 21 and 22 would then have about 10 copies each of rDNA (18 plus 28S) and chromosomes 13, 14 and 15 about 5 copies each. The in situ techniques could now be applied to clinical studies.